masihdindar Flarum uses Laravel's Str::slug
method which itself uses the portable-ascii library.
That library accepts a language parameter that changes how transliteration is done.
Previously, the language would always be "en". Now the forum default language is used instead.
What changes exactly depending on the language provided is difficult to say, because that library does a lot of stuff behind the scenes. From my observations it seems to be mostly to enable language-specific replacement patterns, like the german ß and similar, but it seems like for other languages it enables entire ranges of characters that were previously ignored.
There might still be languages that aren't supported by portable-ascii at all, or maybe the language pack uses a different language code that portable-ascii doesn't recognize.